1. Field of the Invention
The present invention relates to a surge arrester including an elongated insulating housing provided with a top terminal and a bottom terminal and comprising a plurality of electrically series-connected metal oxide varistor blocks arranged in a stack or in several electrically series-connected stacks between the top and the bottom terminals. The invention is primarily intended for surge arresters comprising zinc oxide varistors.
2. Prior Art
In contrast to the varistor blocks in a conventional surge arrester with silicon carbide (SiC) blocks and series-connected spark gaps, the varistor blocks in a zinc oxide (ZnO) arrester (with or without spark gaps) are continuously subjected to a certain operating voltage when the surge arrester is connected into a network which is under voltage. The surge arresters have to be dimensioned in such a way that this voltage stress, to which the ZnO blocks are continuously subjected during normal operation, does not exceed a predetermined value in any place in the surge arrester.
The voltage distribution along the prior art ZnO surge arresters is substantially determined by the self-capacitances of the varistor blocks, by the leakage capacitances of the blocks to ground, and by a grading ring usually arranged at the top of the surge arrester. The primary object of this ring is to improve the voltage distribution which has become uneven because of the leakage capacitances. However, a completely even distribution cannot normally be achieved in such a design, and, accordingly, there is a higher voltage stress at the upper part of the surge arrester than at the lower part.
The active parts of a surge arrester for outdoor use are normally enclosed in a porcelain housing with metallic end flanges. For reasons of manufacturing technique, such a porcelain housing cannot be made too long. Therefore, surge arresters for voltages higher than about 150 kV are normally constructed from two or more surge arrester units mounted on top of each other. In these multi-unit surge arresters, the leakage capacitances of the joint attachments to ground will further strengthen the uneven distribution of the voltage along the surge arrester and thus contribute to the top unit becoming relatively more highly stressed than the other units.
To achieve an acceptable voltage distribution in most long ZnO surge arresters, it is common to connect control capacitors in parallel with the ZnO blocks. However, control capacitors with a sufficiently stable capacitance for this purpose are relatively expensive and result in a noticeable increase in the cost of the surge arrester.
According to another known proposal, the voltage distribution in ZnO surge arresters can be improved without the use of control capacitors by using specially manufactured varistor blocks having self-capacitances which increase successively in a direction from the bottom terminal towards the top terminal. The capacitance of the varistor blocks can be changed by varying, during the manufacture, the addition of antimony trioxide (U.S. Pat. No. 4,276,578). Constructing surge arresters from such specially made varistor blocks, which have several different material compositions, is, however, hardly realistic in view of economical aspects.